The pulmonary interstitium is a pathway that drains microvascular filtrate from the lung. Thus its properties are important to the understanding how normal fluid balance is maintained and the pathogenesis of pulmonary edema. The clearance of interstitial fluid depends on bulk flow caused by differences in hydrostatic pressure and osmotic flow caused by differences in protein concentration. In previous studies, the effects of hydration, protein concentration, fluid viscosity, electric charge, and hyaluronidase on interstitial conductivity were measured. The effects of albumin and hyaluronidase were opposite to those expected from steric exclusion in solutions of albumin and hyaluronan. Recent studies indicated interstitial sieving of albumin and a finite reflection coefficient that increased at low concentrations with normal hydration, in conjunction with free diffusion of albumin. Such a membrane-like property is opposite to the properties of solutions usually assigned to interstitium. A major goal is to describe the interstitium in terms of its membrane properties. The effect of hyaluronidase on interstitial sieving of albumin and the effect of hydration on the reduced diffusive response to hyaluronidase will be studied. The sieving behavior measured in interstitial segments during steady state flow will be supported by studies of the longitudinal gradient in albumin concentration during interstitial cuff growth in liquid-inflated .lungs. These studies will measure steric exclusion of albumin in interstitial cuffs of vessels ranging from arterioles to large vessels.